This invention relates to power plant ventilation and, specifically, to a ventilation scheme for a gas turbine compartment.
A significant contributor to the increase of net simple and combined cycle efficiencies for large industrial gas turbines is the ability to achieve tighter clearances between the tips of the rotating rotor and a surrounding stator. These tip clearances are influenced by deformations of the shape of the turbine casing that supports the stator around the rotor. Circumferential thermal variations in the casing temperature can lead to shape distortions and negatively impact the efficiency of the gas turbine. Accordingly, there is a long felt need for techniques to reduce the temperature variations applied to a casing so as to reduce thermal induced casing deformation.
Large industrial gas turbines using advanced technologies to achieve higher simple cycle and combined cycle efficiencies are sensitive to local environment conditions, such as the temperature surrounding the gas turbine. Industrial gas turbines are typically housed in an enclosure or compartment (collectively referred to as an compartment). The compartment contains, in addition to the gas turbine, a substantial volume of air surrounding the gas turbine. This air is in direct contact with the casing shell of the gas turbine. If the surrounding air becomes excessively hot, the turbine casing may become excessively hot and deform beyond its tolerances.
Ventilation of a gas turbine compartment removes excess heat from the enclosed air and dilutes hazardous gases that would otherwise buildup in the compartment. Compartments for large industrial gas turbines present ventilation hurdles including: the amount of heat to be removed from the air in the compartment may be large; the compartment may be a complex shape with ventilation flow stagnate zones; the turbine hardware in the compartment blocks ventilation flow, and the buoyancy dominated natural convection of air in an compartment may result in thermal gradients applied to the turbine casing. If the compartment is poorly ventilated, the enclosed air surrounding the gas turbine may apply uneven thermal gradients to the casing that deform the casing. The casing deformations may cause excessive clearances or rubs between the rotor tips and stator within the casing.
Modern industrial gas turbines generally require precise rotor-stator clearance control during transient and steady state operation. To achieve this control the turbine shell casing temperature distribution should be maintained within acceptable limits to minimize casing distortion. These clearance limits may vary during different operating modes of the gas turbine. For example, the turbine clearance limits may be increased during cool down transients to improve the hot restart capability of the turbine.
Constant speed fans are conventionally used for compartment ventilation systems. See e.g., U.S. Pat. No. 6,357,221. Temperature triggered switches in the compartment turn the ventilation fan on and off, and provide an alarm when the air temperature in the compartment exceeds the maximum limit. A disadvantage with an air temperature switch is that the actual operating temperature of the casing is not monitored.